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Date: Wed, 07 May 2003 14:14:50 -0400
From: "Julin, Joshua" <joshua DOT julin AT lmco DOT com>
Subject: RE: Cygwin-gdb can't find linker symbol for virtual table
To: "'cygwin AT cygwin DOT com'" <cygwin AT cygwin DOT com>
Message-id: <3888FB26DD0ACF43B7D763789F84289629F341@EMSS03M13.us.lmco.com>
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I've verified that this problem occurs on multiple cygwin installations.
One of them was a brand new installation.

A summary of the problem:
Cygwin-gdb doesn't handle classes with virtual destructors properly.
It prints out a warning "can't find linker symbol for virtual table"
whenever a member variable is printed.
I've verified that this problem does NOT occur on linux.
This problem causes havoc when using insight.

How to recreate the problem in 3 easy steps:
############# step 1: create josh.cpp  ###############
class Object
{
public:
    int x;
    virtual ~Object() { }
};
int main()
{
    Object * obj = new Object;
}

############# step 2: build josh.cpp  ###############
g++ -g -o josh josh.cpp

############# step 3: run gdb  ######################
gdb josh
b main
r
n
p obj->x 

#####################################################
gdb will generate this warning: (which it shouldn't)
warning: can't find linker symbol for virtual table for `Object' value

Thanks for any help,
Josh


> -----Original Message-----
> From: Julin, Joshua 
> Sent: Monday, May 05, 2003 1:29 PM
> To: cygwin AT cygwin DOT com
> Subject: Cygwin-gdb can't find linker symbol for virtual table
> 
> 
> Hey everybody,
> 
> I'm getting an (incorrect?) warning in gdb whenever I try to 
> debug classes with virtual functions.
> When I try to print out a member variable, gdb states 
> "warning: can't find linker symbol for virtual table for 
> `Object4' value".  It then prints out the member variable correctly.  
>   
> This behavior is isolated to my cygwin configuration; I don't 
> get this warning on my Linux box.
> 
> I've searched the internet for this message and can't find 
> any answers...Geoff Alexander did post this question a while 
> back to multiple user groups, but to no avail.  I've searched 
> the gcc man page for 'virtual' but didn't find anything which 
> would make the warning go away.
> 
> This warning message is particularly annoying to me when I 
> use insight, the gdb graphical debugger.  insight causes 
> about 1 to 20 useless dialog boxes to popup whenever I try to 
> 'watch' a member variable.  They all say the same thing: 
> "can't find linker symbol for virtual table".
> 
> Below I show some very simple code that causes this warning 
> on my system.
> I also show what gdb commands I used to cause the warning 
> message to appear.
> 
> Thanks for any help,
> Josh Julin 
> 
> 
> 
> //----------------------------------------------------------------
> //code  (THIS CODE IS FINE, BUT CAUSES WARNING IN GDB)
> //----------------------------------------------------------------
> class Object3
> {
> public:
>     int x;
>     ~Object3() { }                // no virtual foo
> };
> 
> class Object4
> {
> public:
>     int x;
>     virtual ~Object4() { }       // virtual foo
> };
> 
> int main()
> {
>     Object3 * ptr3 = new Object3;
>     Object4 * ptr4 = new Object4;  // when I "watch" ptr4->x 
> I get the warning
> }
> 
> //----------------------------------------------------------------
> // gcc version (ON CYGWIN)
> //----------------------------------------------------------------
> gcc --version
> gcc (GCC) 3.2 20020927 (prerelease)
> 
> //----------------------------------------------------------------
> // gdb session on cygwin  (NOTICE THE WARNING)
> //----------------------------------------------------------------
> GNU gdb 2003-03-03-cvs (cygwin-special)
> Copyright 2003 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public 
> License, and you are
> welcome to change it and/or distribute copies of it under 
> certain conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show 
> warranty" for details.
> This GDB was configured as "i686-pc-cygwin"...
> (gdb) b main
> Breakpoint 1 at 0x4010ae: file josh.cpp, line 17.
> (gdb) r
> Starting program: /home/josh/rearch/tryAgain/josh.exe 
> 
> Breakpoint 1, main () at josh.cpp:17
> 17          Object3 * ptr3 = new Object3;
> (gdb) n
> 18          Object4 * ptr4 = new Object4;
> (gdb) n
> 19      }
> (gdb) p ptr4->x
> warning: can't find linker symbol for virtual table for 
> `Object4' value
> $1 = 0
> (gdb) 
> 
> 
> //----------------------------------------------------------------
> // gdb session on linux  (EVERYTHING WORKS!)
> //----------------------------------------------------------------
> GNU gdb 5.3
> Copyright 2002 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public 
> License, and you are
> welcome to change it and/or distribute copies of it under 
> certain conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show 
> warranty" for details.
> This GDB was configured as "i686-pc-linux-gnu"...
> (gdb) b main
> Breakpoint 1 at 0x8048656: file josh.cpp, line 17.
> (gdb) r
> Starting program: /home/josh/tmp/jjj/josh 
> 
> Breakpoint 1, main () at josh.cpp:17
> 17          Object * ptr1 = new Object();
> (gdb) n
> 18          Object2 * ptr2 = new Object2();
> (gdb) n
> 19      }
> (gdb) p ptr2->x
> $1 = 0
> (gdb) 
> 
> 

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